Conventional steel hot forged parts were given high strength and high toughness by hot forging a steel bar into the shape of the part, then reheating it and patenting it by quenching and tempering. However, the ratio of the patenting costs in the production costs of the part was large, so hot forged non-patented steel eliminating the quenching and tempering patenting has been developed.
In the past, hot forged parts using non-patented steel were produced by heating once to 1200° C. or more and forging at a high temperature of 1000 to 1200° C. or so. However, heating at 1200° C. or more causes the austenite grains to coarsen, while forging at a high temperature of 1000 to 1200° C. or so causes recrystallization after working and results in a coarser structure obtained in the cooling process. Therefore, a hot forged part using non-patented steel, compared with a patented steel part, generally has a smaller proof strength ratio and impact value and a smaller strength difference from the surface layer to the inside, so the machinability dropped along with an increase in the part strength.
To solve these problems, Japanese Patent Publication (A) No. 56-169723 describes to control the suitable ingredient system and cooling rate after hot forging so as to disperse a large amount of in-grain ferrite having MnS cores and as a result make the structure substantially finer grained and improve the fatigue characteristics. However, the structure obtained by this method is still coarse. The amount of increase of strength due to the finer structure is small.
Japanese Patent Publication (A) No. 10-195530 proposes forging by a temperature lower than the conventional forging temperature, that is, by 800 to 1050° C., obtaining a fine ferrite-pearlite structure in the cooling process, and producing a non-patented steel forged part having a higher strength and higher toughness by making the structure finer. However, the crystal granularity of the ferrite obtained by this method is the #10 to #12 or so. The increase in strength due to the finer structure is small.
Japanese Patent Publication (A) No. 2003-147482 further proposes the method of forging by a low temperature of 700 to 800° C., obtaining a ferrite-pearlite structure having an average crystal grain size of the ferrite and pearlite of 10 μm or less by the cooling process, and improving the strength and toughness by the finer structure. However, this method has a forging temperature of a low temperature of 700 to 800° C., so the deformation resistance remarkably increases over conventional forging and the load on the forging machine and tooling becomes greater.
To counter this increase in the deformation resistance due to the reduction of the forging temperature, Japanese Patent Publication (A) No. 2003-155521 proposes a method of production of a high strength forged part characterized by performing a coarse working step of forging the steel to a coarse shape at 1100 to 1300° C., then performing a finishing step of forging the locations where a high strength is required to the final shape at 600 to 850° C. and making the structure transform to a ferrite-pearlite structure in the cooling process so as to make the locations where high strength is required 5 μm or less ferrite grains. However, the tensile strength is a low 600 to 750 MPa. Further, when forging in the practical forging temperature region of 800° C. or more, the yield ratio is 0.82 or less. This is far from quenched and tempered steel.
Furthermore, Japanese Patent Publication (A) No. 2004-137542 proposes a high strength and high yield ratio non-patented steel hot forged member obtained by forging by a forging temperature of a relatively high temperature of 1000 to 1200° C., then cooling to room temperature by a 0.5 to 5° C./sec cooling rate to transform the structure to a ferrite-pearlite structure and further cold working by a degree of processing of 2 to 10%. However, in this method, after forging, a cold working step is added. The manufacturing cost rises by that amount.